1. Field of the Invention
The present invention relates generally to a wireless communication system using multiple antennas, and more particularly, to an apparatus and method for determining a modulation and coding scheme for Adaptive Modulation and Coding (AMC) in a multi-antenna system improving the link performance.
2. Description of the Related Art
Nowadays, many wireless communication technologies have been proposed as candidates to support high-speed mobile communications. Among them, Orthogonal Frequency Division Multiplexing (OFDM) is a sort of multi-carrier modulation scheme, and is now recognized as the most promising next-generation mobile communication technology.
OFDM is a modulation scheme that converts a serial input symbol stream in parallel and modulates the parallel-converted symbol streams using orthogonal subcarriers during transmission. OFDM may be widely applied to digital transmission technologies for high-speed data transmission, including Wireless Internet, Digital Audio Broadcasting (DAB), Digital Multimedia Broadcasting (DMB), and Wireless Local Area Network (WLAN).
The OFDM system has been designed to support high-speed packet data transmissions, and high-speed packet data services may be obtained by efficiently using the transmission bandwidth and by mitigating inter-symbol interference caused by frequency selective fading channels. In addition, the OFDM system makes it possible to transmit packets over frequency selective channels without equalizers.
A Multiple Input Multiple Output (MIMO) system, also known as a multi-antenna system, may provide a significant throughput gain over single-antenna systems by transmitting multiple independent data streams.
Therefore, the recent wireless communication systems have developed into MIMO-OFDM systems provided by combining the MIMO technology with the OFDM technology. The MIMO-OFDM system may guarantee high spectral efficiency and better link error performance by associating with Bit-Interleaved Coded Modulation (BICM).
Meanwhile, an Adaptive Modulation and Coding (AMC) technique is a technology by which a transmitter enhances the link performance by adjusting the transmit power level, channel coding rates and/or modulation orders (or modulation levels) based on the current Channel State Information (CSI). That is, the AMC technique increases throughput (or data rate) in the good channel state and decreases throughput in case of the channel degradation to support efficient data transmission, thus making it possible to increase average throughput. Therefore, a receiver needs a method for accurately estimating and feeding back the link-level performance in a given channel environment.
A combined technology of the MIMO system and the AMC technique differs in the difficulty of its realization according to a type of the receiver in the system.
In MIMO systems with linear equalizers, signal restoration through individual estimation is possible even for the data that is simultaneously transmitted as it is converted into the mathematically same single-antenna signals due to the linear equalizers. That is, if inter-signal interference is cancelled by the linear equalizer, the MIMO system may be considered as a single-antenna system, so the MIMO system may use the link performance estimation technique used in the single-antenna system. Hence, a MIMO system with a linear equalizer may employ the AMC scheme used in the single-antenna system without any difficulty.
However, since a receiver with a linear equalizer is not an optimal receiver in terms of data estimation, the receiver may suffer from performance degradation. Therefore, the MIMO system should use a maximum likelihood detector, which is a nonlinear equalizer, for optimal data estimation.
The maximum likelihood detector exhibits the excellent detection performance compared to the receiver with a linear equalizer, as it simultaneously estimates the received data. Undesirably, however, the maximum likelihood detector may not employ the AMC technique used in the single-antenna system due to its simultaneous data estimation. Therefore, there is a need for a scheme to efficiently apply the AMC technique in the MIMO system with the maximum likelihood detector.